When the accelerator pedal of a vehicle is pressed while the clutch is disengaged, the accelerator is opened to cause the engine to run at “idle” and the engine RPM is stabilized at the engine RPM corresponding to the accelerator opening degree. At that point of time, the driving force generated by the engine and the internal resistance (friction) of the engine are in equilibrium and the engine output torque is 0. That is, the engine does no work for the outside the engine and fuel is wasted. Assuming that the engine runs at idle at the engine RPM of 2000 rpm, a driver can hear loud engine noise and recognize that a considerable amount of fuel is wasted.
The engine does no work for the outside not only during idling caused by disengagement of the clutch as described above but also while the vehicle is in motion. That is, the engine only revolves at an engine RPM corresponding to the accelerator opening degree as in idling and does not contribute to acceleration or deceleration of the vehicle. At this time, the fuel is consumed only in causing the engine to revolve uselessly, which is highly wasteful.